1. Field of the Invention
The present invention relates to apparatus and methods for controlling temperature of printheads in a thermal printer apparatus. More particularly, the present invention is directed to thermal printer apparatus and methods having a thermal print engine that comprises plural printing stations.
2. Description Relative to the Prior Art
In the prior art as represented by U.S. Pat. No. 5,440,328, thermal printer apparatus are known that operate as a single pass, multi-color thermal printer. In such a printer a print engine is provided that comprises a media transport system and three or more thermal print head assemblies. Each of the print head assemblies includes a respective reloadable thermal ribbon cassette which is loaded with a respective color transfer ribbon. Each of the thermal print head assemblies comprises a cantilevered beam, a mounting assembly and a thermal print head having a thermal print line. Each of the print head assemblies has a counterpart platen roller with which a respective print head forms a respective nip and through which the media passes in combination with a respective color ribbon of dye. In lieu of separate platen rollers there may be a single large roller which forms a nip with each of the print heads. The mounting assemblies allow the print heads"" positions to be adjusted so that the mounting assemblies can be pivoted towards and away from the respective platen rollers. In this regard, the mounting assemblies are pivotable between an xe2x80x9cupxe2x80x9d position wherein the print heads are disengaged from the platen rollers and a xe2x80x9cdownxe2x80x9d position wherein the print heads are in biased engagement with the platen rollers.
A problem with thermal printer apparatus of the type described above is the need to reduce waste created when printing must cease due to overtemperature or nonuniform conditions in one of the print heads. Overtemperature conditions may arise due to the requirement of many of the recording elements on a print head for a color to have to record an image at a relatively high density. Thus it is very important that the printer be operating at or below the temperature threshold prior to and throughout the entire printing cycle. It is known that thermal bead temperatures below a certain threshold temperature transfer less amount of dye (color) per transfer unit, usually resulting in low density or light (soft) images. Conversely, thermal bead temperatures above a certain threshold temperature transfer more dye per transfer unit, usually resulting in higher density with darker than desired images. In addition, in order to achieve high-quality photographic looking prints using a thermal printing device (dye diffusions/dye sublimation) it is very important that the distribution along the printing surface or printing line be as uniform as possible when printing a xe2x80x9cflat fieldxe2x80x9d image. Also, it is known that for a typical image, one which may not be a xe2x80x9cflat fieldxe2x80x9d, or gray, the temperature distribution along the bead (or recording line) will vary. Higher temperatures will result with darker image areas and lower temperatures will result in lower density image areas.
It is typical for thermal print engines to preheat a thermal head in some fashion prior to the dye transfer phase of the printing cycle in order to achieve the correct level of dye transfer. Methods of preheating sometimes involve the use of electrically controlled resistive heaters placed between the thermal head and its attached heat sink or, more typically, energizing the recording elements of the thermal head. In either case, the thermal head (bead) temperature is usually determined by the use of a thermistor (or thermocouple) mounted in the thermal head assembly near the thermal bead. Thermistor electrical resistance changes with temperature and is easily monitored by the printer microprocessor.
It is also typical in a thermal printing apparatus to have the thermal head attached to a heat sink (with and without fines) such as aluminum. Some may have a cooling fluid circulated around to maintain proper bead temperature. All for the purpose of minimizing inappropriate amounts of dye transfer associated with the thermal bead being too hot or to cool.
Thermal printing productivity inefficiencies result when the print cycle is delayed due to the heating up or cooling down of the thermal head necessary to achieve the xe2x80x9cstart printxe2x80x9d temperature. In addition, inefficient temperature control management creates undesired density fluctuations within the printer image. Also, and perhaps more importantly, during the printing sequence if the thermal head temperature falls outside the xe2x80x9cnormalxe2x80x9d operating range the printing apparatus must continue to advance the receiver (and donor) media until the entire image has been printed before the next image can be started. It will thus be understood that substantial waste of both paper and dye media can result when the printer apparatus has multiple heads arranged serially along the print path and thus thermal management becomes an important consideration.
It is therefore an object of the invention to improve upon the thermal management in a single pass, multi-color thermal printer.
In accordance with a first aspect of the invention, there is provided a thermal printer apparatus for recording image information on moving receiver media at a print station, the apparatus comprising a ribbon cassette assembly for storing a thermal ribbon having dye, the thermal ribbon including a supply ribbon core and a take-up ribbon core, the cassette assembly including a supply ribbon support for supporting the supply ribbon core and a take-up ribbon support for supporting the take-up ribbon core, the cassette assembly including a wall structure defining a plenum chamber, the plenum chamber having air under pressure; a fan communicating with the plenum chamber for providing air under pressure to the plenum chamber; an elongated thermal print head positionable in engagement with the thermal ribbon for transferring dye from the thermal ribbon to the moving receiver media, the print head having a plurality of recording elements arranged in a main scan recording direction that is perpendicular to an advancement direction of the moving receiver media, the main scan recording direction also being the direction of elongation of the print head; a heat sink associated with the print head and including a series of parallel fins arranged along the length of the print head and the fins being oriented at least generally perpendicular to the main scan direction, and generally parallel to the advancement direction of the receiver media; and wherein the wall structure extends in the direction of elongation of the print head and has one or more openings along the direction of elongation for providing cooling air directed generally to sweep in the direction of the fins so that the cooling air advances generally in a direction generally parallel to the advancement direction of the receiver media at the print station to enhance cooling of the print head.
In accordance with a second aspect of the invention there is provided